Method of increasing the deposition rate of electroless solutions



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m2 ms 93: w m m N mo 8 8 g 8 2 o H. KORETZKY ELECTROLESS SOLUTIONS Filed 001;. 8, 1965 METHOD OF INCREASING THE DEPOSITION RATE OF Dec. 23, 1969 INVENTOR HERMAN KORETSKY f l 1 MI BY 5;. W

ATTORNEYS (w/0v) NOlllSOdI-IO so 31w 3,485,725 METHOD OF HNCREASING THE DEPOSITION RATE F ELECTROLESS SOLUTIONS Herman Koretnky, Poughlreepsie, N.Y., assignor to International Business Machines Corporation, Armonlr,

NFL, a corporation of New York Filed Oct. 8, 1965, Ser. No. 494,125 int. Cl. C23c 3/02; C23b /48 US. Cl. 204-33 11 Claims ABSTRACT 0F THE DISCLOSURE A method for controlling the rate of deposition of metal onto a non-metallic substrate from an electroless plating bath by impressing an electric potential on the surface of the substrate.

The present invention relates to a method for controlling the rate of deposition of metal and metal alloy films from electroless chemical plating baths.

In recent years, considerable effort has been directed toward the development of electroless metal deposition techniques One of the methods which has gained considerable acceptance permits the deposition of metal films on conducting and non-conducting substrates by catalytic reduction of metal Salts from aqueous baths, such as the metal cation-hypophosphite anion type.

Although electroless plating baths have met with some success, certain drawbacks remain. In particular, it has proved to be extremely difiicult to control at desired levels the rate at which metal is deposited from the baths.

In general, in order to achieve a relatively high plating rate, it has been the practice to operate the electroless metal cation-hypophosp'nite anion bath at relatively high temperatures. For example, in electroless solutions containing ammonia as a complexing agent to prevent precipitation of cobalt phosphite, temperatures of about 78 C. are ordinarily maintained. Higher temperatures cause the ammonia to be volatilized from the bath. Acid electroless nickel baths containing no ammonia are usually maintained at temperatures approaching 100 C.

Ultimate decomposition of such baths results from precipitation of insoluble metal phosphites. Since these phosphites have a reverse solubility curve, i.e., they are less soluble at higher temperatures, it is desirable to maintain the bath at the lowest possible temperature to retard their precipitation and extend the life of the bath.

However, operation at lower temperatures normally is attended by slower plating rates.

Further, it has been observed that the rate of deposition of metal films from electroless solutions has a significant eifiect on the properties of the resulting metal film. For example, where magnetic metal films are electrolessly deposited, the magnetic and other properties of the film may be modified by increasing or decreasing the rate at which it is deposited.

Therefore, the general object of the present invention is to provide a method for controlling the rate of deposition of metals from electroless solutions.

A more specific object is to provide a method for increasing the rate of metal deposition from electroless bahs. A collateral object is to provide for an increased rate of deposition from electroless baths operated at relatively low temperatures, thereby extending the life of such baths.

Another object of the invention is to modify the properties of metal and metal alloy films deposited from electroless solutions by selectively increasing or decreasing the rate of deposition of the film.

nited States Patent 0 Other objects and advantages of the present invention will become apparent in view of the following detailed description of the invention. The detailed examples which appear below represent preferred embodiments of the process and illustrate the best mode which has been contemplated for carrying out the invention.

The advantages of the invention are further shown in the accompanying drawing, the single figure of which is a graph which is a plot of the rate of electroless metal deposition, in A. of film thickness per minute, (A./ min.) vs. current density, in amps per square foot (a.s.f.).

In accrdance with the present invention, it has now been discovered that the rate of metal deposition from electroless solutions may be selectively controlled by impressing a small electric potential on the substrate which is to be plated. In particular, it has been found that by impressing a small negative potential on a substrate an increase in plating rate is realized when the substrate is contacted with standard electroless metal solutions. Also, a small positive potential impressed on the substrate will reduce the rate of metal deposition.

In the preferred practice of the invention an increase in the rate of metal deposition is achieved by immersing the substrate in a conventional electroless plating solution and impressing on the substrate a small negative potential by placing it in a direct current circuit. A positive electrode is also placed in contact with the solution and the positive electrode and substrate are connected to a source of direct current to complete the circuit. The vessel in which the electroless solution is contained may itself serve as the positive electrode.

In order to reduce the deposition rate, a positive potential is impressed on the substrate by placing it in a direct current circuit comprising the substrate, a negative electrode in contact with the bath and a source of electric current.

The electroless baths which may be employed in the practice of the invention include standard electroless metal depositing solutions, such as the well known metal cation-hypophosphite anion type. Such baths conventionally contain various amounts of stabilizers, pH modifiers and the like.

The substrates onto which electroless deposition may be made by this process include metals, non-metals, conductors and non-conductors. For example, conductive substrates of copper and aluminum and non-conductive substrates of glass and plastic may be coated utilizing the present invention.

Likewise, the invention is applicable to the deposition of any metal or alloy capable of being deposited from electroless solutions.

By way of illustration, the invention has application in the deposition of metal films, such as thin ferromagnetic films of metals from the group consisting of cobalt, nickel and iron and their alloys, onto both conducting and non-conducting substrates. The substrates are ordinarily sensitized prior to contact with the electroless plating solution by being treated with aqueous solutions containing tin and/or palladium ions. Satisfactory sensitization procedures are disclosed in U.S. Patents 2,702,253, Bergstrom and 3,142,582, Koretzky et al.

In practicing the invention, the sensitized substrate is immersed in an electroless plating solution containing metal cations, and hypophosphite anions. The solution may also contain various complexing agents and pH control agents or a combination of agents as is well known in the art. Where it is desired to increase the'rate of deposition, a negative potential is then impressed on the substrate by placing it in a direct current circuit. A positive potential may be introduced into the system by an electrode immersed in the plating bath or by making an electrode of the container in which the electroless bath is placed.

A very small direct current, sufiicient to enhance the rate of deposition is then passed through the substrate. As will be shown more in detail in connection with the examples, improvement is realized with very small microcurrents up to about 60 a.s.f. and particularly good results are observed when a microcurrent of only up to about 10 a.s.f. is impressed on the substrate.

Where the substrate is a non-conductor, the electroless deposition is first permitted to proceed to such an extent that sutficient metal is deposited on the surface of the substrate to carry the microcurrent.

The following specific examples will assist toward a full appreciation and understanding of the present invention.

EXAMPLES 1-3 Vessel Vessel Vessel No. 1 N o. 2 No. 3

00804.7Ha gmJl 34. 55:2 34. 53:2 34. 5i: NBHzPOLHZO gun/1. 20:1:1. 5 20:111. 5 20i1 5 NBzHPOxjHzO (NH4)2SO4 66 60 NaaGsHraOmZHzO 35 35 Suflicient NH OH was added to each vessel to bring the pH to 7.8 at room temperature and the temperature of each vessel was maintained at 78 C. No electrical connection was made to the stainless steel vessel or to the plastic substrate in vessel No. 1. In vessels Nos. 2 and 3, the substrate and the stainless steel vessel were connected in a direct current electric circuit, the substrate having a negative potential with respect to the stainless steel vessel.

The substrate in vessel No. 1 was plated for 6 hours without any current. The substrate in vessel No. 2 was also plated for 6 hours and a total of 3480 coulombs was passed through the substrate during that period. The substrate in vessel No. 3 was plated for 4 hours and 20 minutes and a total of 3480 coulombs was also passed through the substrate during that time.

The samples of plastic substrate were then removed from each of the three vessels and were weighed.

Subtracting the original weights of the substrates, which were initially of the same weight and area, the sample from vessel No. 1 showed a total weight gain of 1.55 gm. The sample from vessel No. 2 showed a total weight gain of 4.45 gm. and the sample from vessel No. 3 showed a total weight gain of 3.25 gm. The amount of the weight gain of the samples from vessels Nos. 2 and 3 attributable to an electroplating effect is about 1.10 grams, assuming 100% efiiciency. Subtracting this amount from the total weight gain of samples Nos. 2 and 3,

it is seen that a gain of 3.35 g. and 2.15 g. is achieved by electroless deposition in vessels Nos. 2 and 3, respectively.

The plating rate in angstroms per minute in the three vessels was as follows: No. 11500, No. 23260 and No. 32920.

The average current density during the plating in vessel No. 2 was 4.64 a.s.f. and in vessel No. 3 it was 6.4 a.s.f.

The foregoing examples clearly demonstrate two points. First, passage of a small microcurrent through a substrate greatly increases the rate at which metal was deposited from the electroless bath. The improvement obtained is on the order of double the efiiciency of the deposition from a bath unaided by the passage of a microcurrent through the substrate. Secondly, the examples illustrate that the improvement is not simply the result of adding an electroplating effect on top of an electroless deposition. If this were the case, the higher current density as employed in the Example 3 should have resulted in a further increase in the rate of deposition. On the contrary, the results in Example 2, utilizing a lower current density, reflect a higher rate of deposition than in Example 3.

To illustrate further that the present invention is based on a unique phenomenon, not explicable as the addition of electroplating to electroless deposition, additional depositions were conducted with three standard electroless hypophosphite baths, one an alkaline cobalt electroless bath, the second an acid nickel hypophosphite bath and the third an alkaline nickel hypophosphite electroless bath. A number of depositions were conducted varying the current density from 0 to 62 a.s.f. and the rate of deposition at each current density was recorded. The data appears in the following table:

TABLE Alkaline Acid Alkaline Current density, a.s.f. cobalt nickel nickel Plating rate in A./rnin.

l, 500 2, 000 700 3, 720 3, 000 3. 100 3, 600 3, 000 2, J00 3, 260 3, 000 .2, T 2, 920 2, 000 l 625 2, 825 2, 750 2, 525 1, 825 2, 2, 050 1, 225 1, 775 l, T75

The results are illustrated graphically in the accompanying drawing. In this graph the vertical axis is graduated in angstroms of film thickness per minute representative of the plating rate and the horizontal axis is graduated logarithmically in a.s.f. units representing current density on the substrate.

The results of the deposition from the alkaline cobalt bath are identified by circled points on curve A of the graph. The results of the depositions with the acid nickel bath are enclosed in triangles on curve B of the graph. The results from the alkaline nickel bath are enclosed in squares on curve C of the graph. No measurements were taken below 0.15 a.s.f. so the curves A, B and C are indicated by partly broken lines leading back to values observed with no impressed potential.

Lines D, E and F represent the base rates of deposition from the electroless baths A, B and C, respectively where no potential is impressed on the substrate and illustrate clearly the vast improvement in rate of deposition which may be realized.

It will be seen that with no superimposed potential difference the base rates of metal deposition in angstroms per minute for Bath A, alkaline cobalt, Bath B, acid nickel. and Bath C, alkaline nickel, are 1500 (Line D), 2000 (Line E), and 700 (Line F), respectively. However, the rate of deposition is dramatically increased to 3720, 3000 and 3100 A./min., respectively, when a current density of only 0.15 a.s.f. is impressed on each of the three substrates in the same manner as described in Examples 2 and 3. The comparison of such deposition rates, represented by curves A, B and C, with the base rates, without impressing a potential on the substrate, represented by Lines D, E and F. strikingly illustrates the large magnitude of the improvement achieved by the present process.

As the current density is increased slightly to 0.30 a.s.f. and 2.85 a.s.f., the improvement in rate of deposition decreases, but still otters considerable advantage over deposition Without any negative potential being impressed on the substrate.

The greatest advantage is seen to be gained at very small current densities, generally below 10 a.s.f.

Likewise, at current densities on the order of 28.8 a.s.f. and 30.5 a.s.f. the deposition rate is higher than the rate without a negative potential on the substrates, but is considerably below the optimum high rates achieved at very low current density.

Finally, as the current density is raised to a level on the order of about 60 a.s.f. and 62 a.s.f., little, if any, improvement in plating rate is realized.

It is interesting to note that initially the acid nickel bath, Bath B, and the alkaline nickel bath, Bath C, exhibit widely difierent deposition rates where no current is impressed on the substrate. However, on application of the same small microcurrent to substrates in each bath, the deposition rates closely approximate one another.

This unexpected result gives support to one theory advanced to explain the phenomenon underlying the invention. It is theorized that negatively charged ligands formed in the bath tend to retard deposition by blocking the approach of metal ions to the substrate. This is a factor especially in the alkaline nickel bath which requires substantially more complexing material to prevent precipitation than does the acid nickel bath. However, impressing even a small negative potential on the substrate is believed to dissipate the negatively charged ligands and permit the deposition to proceed at approximately the same rate in the baths.

As noted, the benefits of the process are achieved, as can be seen from the table, with very small current densities it being only necessary that the magnitude of the potential be sufiicient to alter the deposition rate from that obtained where no potential is impressed on the substrate. Optimum values can be determined experimentally for particular baths, substrates and conditions.

It will be apparent to those skilled in the art that modifications may be made in the process disclosed herein without departing from the spirit or scope of the invention as expressed in the following claims.

What is claimed is:

1. A method for controlling the rate of deposition of metal onto a substrate in a metal cation-hypophosphite anion electroless plating bath comprising continuously impressing an electric potential of less than a.s.f. on the surface of said substrate, said potential being of sufiicient magnitude to alter the rate of deposition from the rate observed when no potential is impressed on said substrate.

2. The method of claim 1 wherein said potential is a negative potential which increases said rate of deposition.

3. The method of claim 1 wherein said potential is a positive potential which decreases said rate of deposition.

4. The method of claim 1 wherein said potential is produced by connecting said substrate in a direct current circuit.

5. The method of claim 2 wherein said potential is produced by connecting said substrate in a direct current circult.

6. The method of claim 3 wherein said potential is produced by connecting said substrate in a direct current circuit.

7. The method of claim 1 wherein said electroless bath is one capable of depositing a film of a magnetic metal selected from the group consisting of cobalt, nickel, iron and their alloys.

8. The method of claim 2 wherein said electroless bath is one capable of depositing a film of a magnetic metal selected from the group consisting of cobalt, nickel, iron and their alloys.

9. The method of claim 3 wherein said electroless bath is one capable of depositing a film of a magnetic metal selected from the group consisting of cobalt, nickel, iron and their alloys.

10. The method of claim 5 wherein said electroless bath is one capable of depositing a film of a magnetic metal selected from the group consisting of cobalt, nickel, iron and their alloys.

11. The method of claim 6 wherein said electroless bath is capable of depositing a film of a magnetic metal selected from the group consisting of cobalt, nickel, iron and their alloys.

References Cited UNITED STATES PATENTS 3,303,111 2/1967 Peach 204-48 3,264,199 8/1966 Fassell 20438 3,178,311 4/1965 Cann 117-130 2,644,787 7/1953 Bonn 20443 2,690,401 9/1954 Gutzeit et al 11747 JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner U.S. Cl. X.R. 

